1. Field of the Invention
The present invention relates to an apparatus for the continuous production of a polyvalent metal by electrolysis of a halide of said metal dissolved in a bath of at least one melted salt.
2. Description of Related Art
The term polyvalent is here understood to refer to any metal, whose halide is likely to have when dissolved in a bath of melted salts several stable valency states. It can e.g. be titanium, zirconium, niobium, uranium, hafnium, vanadium, tantalum or rare earth metals.
The Expert knows that it is possible to bring about the deposition of a metal by introducing one of its derivatives, such as e.g. a halide, into a bath of melted salts and subjecting it in its simplest form to the action of two electrodes connected to the poles of a direct current source, the halogen being released at the anode and the metal deposited at the cathode. This fusion electrolysis procedure has undergone a large amount of research leading to the development of several processes, which differ as regards the bath composition, the halide concentration of the bath, the physical and chemical state of the halide used, the current density values applied to the electrodes and the production of numerous models of apparatuses differing as a result of their structure and shape, particularly with respect to the electrodes, the halide injection systems and the deposited metal recovery systems.
Most of the presently known apparatuses have a single cell and operate with chloride release at the anode, which makes it possible to place a porous metal diaphragm between the anode and the cathode so as to ensure that the halogen given off does not reoxidize the products resulting from the reduction of the halide by the current and in particular the metal which can be attacked and then forms a heterogeneous deposit liable to contain bath inclusions.
In a certain number of apparatus, the introduction of the halide into the cell takes place via complicated dissolving means and/or means which are difficult to use industrially.
In other apparatuses, the operating conditions are such that the bath is completely saturated with an intermediate valency halide. It is then necessary to use complex stirring systems and to strictly control the temperature in order to prevent any precipitation or spontaneous dismutation of the halide. In addition, certain reduced halides decompose with the formation of sludge, so that the cell has to be periodically stopped for cleaning purposes.
Thus, for example, during the production of titanium by electrolysis, if the metal has a valency equal to or above 2.3 in the halide, the metal deposit becomes spongy and very fine. However, if the said valency is equal to or below 2 and the metal has a weight concentration in the bath equal to or above 5%, sludge formation occurs.
This example shows that it is necessary for the purpose of obtaining a good quality metal under correct cell operating conditions, to very precisely regulate the valency of the metal dissolved in the bath, together with its concentration. However, this could only be obtained, accompanied by other conditions, through the introduction into the cell of a regular halide flow, linked with the deposition rate, in order to maintain the desired concentration, the application to the electrodes of electrical conditions such that they permit an equilibrium of the valency degrees, an appropriate polarization of the diaphragm so as to prevent any metal deposits leading to a clogging or also any attack or action which would lead to perforations or disturbances of the valency state due to the halogen escaping into the cathode compartment.
On an industrial scale, it is very difficult to satisfy these conditions. A large number of scientists have attempted to overcome the problems involved. Thus, e.g., British Patent No. 1 579 955 claims a process for the electrolytic deposition in particular of titanium in an alkaline earth and alkali metal chloride bath, where use is made of a single cell, where:
(a) use is made of a first and a second cathode and an anode;
(b) a salt of the metal at a higher valency is introduced;
(c) the salt is electrolytically reduced to a lower valency on the first cathode;
(d) a mechanical displacement takes place of the lower valency salt from the first cathode, and it is dispersed in the bath;
(e) the metal is electrolytically deposited on the second cathode; stage (c) being performable in a deposition cell separate from the reduction cell, or in the same cell.
This process uses a bath in which, after reduction, the salt formed is at a concentration higher than its solubility. Although this supersaturation is theoretically advisable, in practice, it leads to a decomposition of the salt or to a dismutation during which the titanium precipitates in such a way that sludge formationn occurs. This dismutation modifies both the salt bath concentration and the value of its mean valency, so that, as stated hereinbefore, a poor deposit may be obtained.
In addition, the mechanical displacement of the salt from the cathode requires difficulty usable mechanical means. Furthermore, the dispersion of the salt in the bath involves the use of powerful stirring means in order to obtain an appropriate homogeneity.
Finally, the deposition cell contains an anode where the halogen is given off in the vicinity of the deposition cathode and between these two electrodes is placed a diaphragm, which leads to the disadvantages referred to hereinbefore.